The invention relates to an automatic valve commonly employed on municipal water utility systems, particularly to an automatic valve designed to control downstream pressure between selected set-points depending on the operation of the valve at a particular time.
Automatic pressure reducing valves are used in water distribution systems to reduce pressure to a pre-determined value or set-point that is adequate but does not expose normal components, such as household hot water tanks, to overpressure. The set-point is determined according to the use of the valve, and is a pressure downstream from the valve which the valve attempts to maintain constant for all flow conditions and is termed downstream pressure. Water distribution systems experience significant losses of drinking water due to leaks, and as volume lost through leakage is directly related to pressure in the system, the set-point is determined to provide minimum pressure that meets criteria of the water utility, particularly under maximum or "peak" demand conditions which can occur when fire is being fought. The pressure required for peak demand is usually significantly higher than that required for "off-peak" or typical night time conditions.
It is known to provide a valve which can vary the set-point and thus downstream pressure from the valve, so as to maintain the set-point relatively low for off-peak or night time operation, thus reducing losses in the pipe, and yet can raise the set-point during high flow demand for daily peak or special conditions such as occurring during fire fighting. To the inventor's knowledge, there are two main types of automatic flow control valves with variable set-points, namely, electronically controlled valves and "flow-driven" valves. The electronically controlled valves are relatively complex and require an electrical power source, such as batteries or access to utility power lines, and this increases installation costs and servicing costs and such valves are used in limited numbers. The flow-driven valves use fluid pressures to control actuation of the main valve, and thus are independent of external power sources and can be used in essentially any location.
One type of such valve is supplied by Bermad, a corporation of Evron, Israel. This valve has a flow sensor responsive to differential pressures which are applied on opposite sides of an actuator diaphragm. A rigid push rod transfers diaphragm movement to a controlling valve member of a pilot valve which in turn determines control pressure which actuates the main valve. The actuator diaphragm is immersed in fluid which severely limits movement thereof, and this restriction limits movement of the push rod and valve member thus reducing valve sensitivity. To the inventor's knowledge, such valves can be difficult to adjust, and are prone to instability resulting in wide pressure fluctuations. In addition, if the flow sensor is subjected to unusually high flow, the main valve can be opened excessively, resulting in excessively high downstream pressures which could cause damage to equipment and household appliances exposed to such pressure.